This invention relates generally to inductors of compact design and, more particularly, to inductors that can be fabricated in printed circuit form. Conventional inductor technology for high-frequency (HF) filters, typically in the 10 s of Megahertz (MHz), require filter inductor values as high as one or two microhenries (uH). Physical realization of inductance can be achieved in a number of ways, but is largely dependent upon the inductor application; the frequency of operation and available physical space. In general, achieving a larger value of inductance requires a physical inductor with a larger volume. A larger volume also results in an inductor with a higher quality factor (Q). The Q of an inductor may be thought of as the ratio of the reactance of the inductor (measured by the product ωL, where ω is angular frequency and L is the inductance) to the resistance of the inductor. Q is also defined, in the context of a tuned circuit or a filter, as the ratio of bandwidth to center frequency.
Filters perform the function of selectively transmitting or rejecting electrical signals over a desired band of frequencies. Passive filters typically include a combination of inductors and capacitors selected to obtain the desired performance characteristics. A figure of merit for any filter is defined by it's ability to reject certain frequencies and pass others. For a bandpass filter, the ratio of filter bandwidth to center frequency is commonly referred to as the Q of the filter. The reciprocal ratio expresses the percent bandwidth (% BW) of the filter. For narrow bandwidth filters (i.e., filters with high Q or low % BW), the circuit element (inductor or capacitor) with the lowest Q in general determines the narrowest achievable bandwidth and shape factor of the filter. For filters operating in the HF band, filter capacitances typically have capacitance values less than 200 pF (picoFarads). Because these capacitance values are routinely available with high unloaded a's (greater than 200), in most HF filters inductors are the circuit elements that limit the performance of the filters. Therefore, the design of inductors to achieve high unloaded Q values is of critical importance in the design of HF filters.
Self resonance is also very important for filter design because frequency rejection is predicated upon a specific value of inductance at a given frequency; and operating an inductor at or near self resonance increases or decreases this value, and an inductor may “act” as a capacitor beyond the self resonance point. This is true also of capacitors, which may become inductive beyond the self resonance point. In summary, then, high Q and high self resonant modes are key figures of merit for inductors.
Although inductors with ideal electrical characteristics are easy enough to construct if there is no volume limitation, a competing goal is to make inductors as compact as possible for use in radio-frequency (RF) communication devices. Consistent with this goal, inductors have been designed or proposed for use in conjunction with printed circuit boards. Generally, there are two types: (a) printed/planar inductors in the form of metal traces in spiral, octagonal or other patterns, or (b) wire-wound inductors of cylindrical/solenoidal or toroidal shape, which are typically encapsulated and surface mounted on a printed circuit board. Although some of these structures may be designed to have a relatively high Q value, a common drawback to the latter structures is that surrounding metal structures acting as RF or AC return paths effectively lower the Q value, referred to as “de-Qing.” This phenomenon is also referred to as the “lid effect” when filters are placed in metal cavities. This undesirable effect typically arises because of parasitic capacitances to ground. Currents are induced in these structures, reducing the effective inductance, and also reducing the components' Q value.
The goal of achieving both a compact inductor volume for large inductance and Q values has remained largely elusive to designers. Larger inductance values can be realized either by using a large cross-sectional area for each turn of a conductor or wire forming the inductor, or by using an increased number of turns. In addition, embedding a high permittivity material, such as a ferrite mixture, in the magnetic flux field of the inductor further increases the inductance value. Unfortunately, increasing the cross-sectional area or the number of turns is not consistent with the goal of compactness, and one drawback of adding high permittivity material is that the increase in inductance value is obtained at a cost of a lower effective Q factor.
Accordingly, there is still a need for an alternative approach to the construction of a more compact inductor that has desirable electrical properties and without the drawbacks of the prior art. The present invention satisfies this need.